Arginine ammonification assay as a rapid index of gross N mineralization in agricultural soils

Seasonal dynamics of in situ gross nitrogen (N) mineralization rates were m easured using the N-15-NH4+ isotope dilution method in a Danish soil subjec ted to four different agricultural practices (set aside, barley, winter whe at and clover). Results were compared to arginine ammonification in the soi l samples measured as NH4+ production following addition of excess (1 mM) a rginine. In the set aside, barley, winter wheat and clover soils the averag e annual rates of gross N mineralization (0.29, 0.60, 1.34 and 1.75 mug NH4 +-N g(-1) day(-1), respectively) and arginine ammonification activity (0.21 , 0.55, 0.88, and 1.33 pg NH4+-N g(-1) h(-1), respectively) were well corre lated. Furthermore, the seasonal variations of gross N mineralization and a rginine ammonification activities were very similar, showing rapid response s to rainfall and generally higher activities in wetted soils. As tested in the laboratory, the arginine ammonification activity correlated well with heterotrophic microbial respiration activity (CO2 production) in soil sampl es and further displayed a simple, one-component Michaelis-Menton kinetics with a high affinity for arginine (K-m value of 48 muM +/-5 muM) as determi ned from non-linear parameter estimation. This indicated that arginine ammo nification activity was primarily due to microorganisms, and the activity w as also shown to be at a minimum in sterile soil samples. All evidence thus supported that our standard assay of arginine ammonification activity prov ides a good index of gross N mineralization rates by the microorganisms in soil under in situ conditions.